When conceiving an oxygen injection lance, special consideration must be given to a certain number of parameters. Among these parameters the two most important factors are:
the velocity of the jet at the exit of the tuyere expressed as a Mach number, i.e. relative to the speed of sound which reflects the impact strength of the jet on the surface of the metal bath, and PA1 the gas throughput, the optimum of which is dependant on the volume of the metal bath in the vessel and on the specific metallurgical effect to be achieved during a given phase of the refining operation.
In order to create a supersonic gas jet, a specially profiled part of the gas conveYing duct, called the tuyere, is located in the lower part of the lance body and comprises, in the direction of the gas flow, a converging part, a cylindrical throat and a diverging part. Such a tuyere is known under the name of Laval tuyere. Calculation shows that the Mach number varies is a function of the pressure of the gas supply source at the entry of the lance. The optimum throughput is a function of the gas pressure at the inlet of the tuyere and of the diameter of the throat of the converging portion of the tuyere.
It appears clearly that the two parameters, which are the Mach number and the gas throughput, are both depending upon the geometric configuration of the tuyere and cannot be varied one independently from the other. This implies that it is for example not possible to operate the refining either with a hard jet at a high Mach number and a reduced gas flow or else with a soft jet at a low Mach number and a high gas flow with the help of one same lance conceived to allow a large gas throughput, without deviating in one direction or in the opposite direction away from the optimum parameters resulting from the geometric configuration of the lance. For example, if the lance is operated at higher flow rates and ejection velocities as those for which the lance has been designed, shock waves are created in the interior of the vessel and in the vicinity of the mouth of the lance. As a result hereof the characteristics of the jet are degraded and the wear of the lance mouth is increasing rapidly.
On the other hand, the metallurgist has very often to face situations where he wants to be able, during certain phases of the refining, to blow onto the metal bath soft vertical gas jets with a high flow rate. This blowing practice is for example, recommended if during hot metal refining a strongly oxidized slag has to be obtained. It happens just as frequently that refining would have to be operated with a vertical gas jet which is hard and penetrating, the flow rate being however low. Such an operating procedure is indicated if the total volume of oxygen to be supplied at given moments to the hot metal in the converter has to be small in order to avoid oxidizing of the slag while a strong decarburizing of the metal has to be held upright. So during a same refining cycle, diametrically opposed blowing conditions, i.e. a hard jet for a small gas throughput or a soft jet for a large gas throughput, might be required.
An oxygen top blowing lance including a Laval tuyere has been described in the U.S. Pat. #4,730,784, the disclosure of which is incorporated herein by reference, which teaches the concept of varying independently one from another and within given limits the Mach number and the optimum flow rate of a main stream, the characteristics of this stream being additionally controlled by a secondary gas envelope. To this end, the pressure of the two gas streams can be modified independently and the effective outlet area of the primary stream can be varied. An increase or a reduction of the cross section of the main duct is achieved with the help of an extremely tapered needlelike member movable within the central profiled tuyere along the axis thereof. This rather sophisticated lance is not very easy to operate and it appeared moveover desirable to further enlarge the limits within which the characteristics of the main gas stream can be changed.